Method of forming phosphate coating on zinc



United States Patent 3,240,633 METHOD OF FORMING PHOSPHATE COATING 0N ZINC Lawrence Paul Gowman, Detroit, and James I. Maui-er, St. Clair Shores, Mich, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed June 4, 1962, Ser. No. 199,619 Claims. (Q1. 148-615) The present invention relates to an improved method for forming adherent, corrosion resistant, deformationprotective coatings on Zinc surfaces, and to materials for forming such coatings.

The method of this invention more particularly concerns a procedure for forming phosphate coatings on zinc surfaces which coatings are improved in the respect that they are uniquely adapted to protect zinc surfaces that are to be thereafter deformed, such as by bending, stamping, pressing or punching, into an article of preselected shape. Additionally, this invention includes the provision of means to modify heretofore known phosphate coating solutions for zinc surfaces so that the solutions may be conveniently and simply operated to produce a uniform, pre-selected coating weight.

Aqueous acidic solutions which are capable of forming phosphate coatings on zinc surfaces are now well known and certain of those solutions have achieved widespread commercial use. Such solutions typically include the phosphate ion, the zinc or manganese ion and one or more of the nickel, cobalt, copper, nitrate, nitrite, fluo borate or silicofiuoride ions. Although this art has been able to form phosphate coatings on zinc since about 1917 and successive discoveries of the effects on the coating ability of such solutions of the nitrate, copper, nickel, fluoborate, and silicofluoride ions were made through the years, the best present day formulations are inadequate in certain respects. For example, certain types of paint applied over the heretofore known coatings develops a roughness which is referred to as hazing, and the gloss is not of the highest order. Moreover, painted surfaces subjected to bending do not resist flaking, cracking and the like to the commercially desired degree.

It is, therefore, the primary object of this present invention to provide a process for coating zinc surfaces which is especially adapted for use as an adherent protective coating during metal deformation processes.

Another object of this invention is to provide an improved process for forming adherent phosphate coatings on zinc surfaces which are suitable to protect the surface during forming to final preselected shape and sufficiently adherent to serve as a base for paint, without further treatment, after said deformation.

A further object of this invention is to provide a process for coating zinc which has the ability to form a phosphate coating of uniformly increasing weight as the temperature of operation is increased within the range of about 120 F. to about 180 F.

Another important object of this invention is to provide materials suitable for use in formulating the aqueous solutions which are usable to form the improved coatings of this invention.

A still further object of this invention is to provide a process which is suitable to continuously form a phosphate coating on zinc strip surfaces, which coating is a superior base for paint than heretofore known coatings.

In accordance with this invention it has been found that the above objects may be achieved by modifying the heretofore known solutions to include the ferric ion in an amount in the range of about 0.0015% to saturation. In the preferred form of the invention which modifies the heretofore known solutions to one which is suitable to "Ice form a controlled, preselected coating weight, the solution is modified so that it is saturated, or near-saturated with the ferric ion.

Broadly stated, the aqueous acidic coating solutions which achieve the above objects contain about 0.5% to about 4% of the phosphate ion, at least a sufficient quantity of the zinc or manganese ion to form dihydrogen phosphate with the phosphate ion which is present, about 0.01% to about 1% of the nickel or cobalt ions or a similar concentration of a mixture of the nickel and cobalt ions, about 0.03% to about 1% of the fluoride ion and the ferric ion in an amount greater than about 0.00l5%.

The coating solutions, materials and the method of this invention are useful in forming coatings on zinc surfaces which are free of iron such as galvanized surfaces produced by hot dipping or electro-galvanizing, predominantly zinc surfaces containing small quantities of aluminum, pure zinc, die-cast Zinc, etc.

The fluoride ion may be introduced in the form of hydrofluoric acid, as fluoborate or silico fluoride, but the provision of the required amount of fluoride ion in the form of fluoborate or silico fluoride is preferred. When the fluoride ion is introduced as hydrofluoric acid and the solution does not contain other ions which at least partially form complexes with the fluoride ion, the upper concentration of the fluoride must be maintained well below the upper indicated limit of about 1%, for example, not more than about 5%. The fluoborate ion and the silico fluoride ion provide superior results when used on continuous hot dip zinc surfaces, and since they are readily commercially available and provide both the necessary fluoride concentration and concurrently supply the other beneficial ions it is much more desirable to formulate the compositions with one or both of them as the starting materials rather than hydrofluoric acid.

The ferric ion may be introduced into the solutions in the form of any of the available ferric salts which contain anions that are not detrimential to the coating forming ability of the solution, for example, ferric acid phosphate, ferric nitrate, ferric chloride, ferric fluoride, ferric fluoborate or may be added as the ferrous ion if an oxidizing agent is also added which will oxidize the ferrous ion to the ferric state, such as hydrogen peroxide, permanganate, nitrite, etc.

The nickel or cobalt or nickel and cobalt ions may be introduced as salts such as the sulfate, phosphate, carbonate or nitrate salts, preferably as the carbonate salt.

The incorporation of the ferric ion in the aqueous acidic solutions of this invention is effective to substantially reduce the coating weight which is obtained over a wide range of solution acidities. It has been found that solutions having total acid values in the range of about 10 to about points are effective to form adherent protective coatings and are improved by the addition of the ferric ion. Points of total acid refers to the number of ml. of N/ 10 NaOH required to titrate a 10 ml. sample of the solution to a phenolphthalein end point.

Coating solutions which contain only trace or impurity concentrations of the ferric ion, such as, for example, 0.000l% to about 0.0008% do not appear to have any appreciable effect on the coating forming ability of an otherwise properly compounded solution, but a substantial effect is obtained when the ferric ion content reaches about 0.0015%. As the ferric ion content is increased above about 0.00 15% upwardly toward the saturation value, the coating which is produced at any given temperature and time is decreased in weight, with the mini mum coating weight occurring at the ferric ion saturation value. The decrease of coating Weight is not entirely linear with increase in ferric ion concentration but is reasonably linear and the total decrease between 0.0015% and saturation is usually no greater than the decrease between zero ferric ion and about 0.0015% ferric ion. For example, an operating solution containing 0.27% zinc, 0.22% N 0.98% P0 0.21% fluoride as SiF 0.23% Ni, 0.0001% Fe+++ and having a total acid of 26 points produced an adherent coating on hot dip zinc having a weight of about 515 mg./ sq. ft. after 15 seconds contact at 150 F. In the same bath operating under the same conditions but modified to contain 0.00l5% ferric ion, the coating was reduced to about 300 ing/sq. ft.; at 0.0026% ferric ion the coating was about 250 mg./sq. ft.; at 0.0042% ferric ion the coating weight was about 200 mg./ sq. ft. and at saturation the coating weight was about 170 mg./sq. ft.

As the total acid value increases from about points upwardly toward 110 points, the ferric ion saturation value increases from a value of about 0.0015% up to about 0.023%. In the higher total acid solutions the coating weight obtained from a ferric ion free solution is much higher than is obtained from a less acid solution, and the total coating weight reduction which occurs from saturating the solution with ferric ion is much greater. containing 0.0001% ferric ion defined otherwise as to chemical constituency above, when applied for seconds at 150 F. to electrogalvanized zinc produced a coating weight of 384 mg./ sq. ft whereas, the same solution saturated with ferric ion produced a coating weight of about 140 mg./sq. ft. In a bath of the same general type but modified to have a total acid value of 110, a coating weight of 792 mg./sq. ft. was formed on hot dip zinc after a 15 second contact at 150 F. The same bath saturated with ferric ion formed a coating weight of 183 mg./sq. ft. after 15 seconds at 150 F.

As the lower limit of about 10 points total acid is approached the coating weight difference is small between that obtained in the ferric ion free and ferric ion saturated solutions, but the coatings obtained from the ferric ion containing solutions are finer crystals, harder and somewhat darker in appearance.

It has also been observed that the hot dip zinc, electrogalvanized zinc and other commercial zinc materials obtained from different zinc product manufacturers have somewhat variant phosphate coating characteristics That is, a single solution of this invention used for coating zinc from different sources under identical application conditions will normally produce somewhat different weight coatings. In fact, the coating-forming characteristics of the zinc material has some variation from batch to batch when it is obtained from a single supplier. Nevertheless, the above described coating weight reductions apply, in general, to the entire range of zinc materials which are available to the trade on an approximate percentage of reduction basis which relates the ferric ion free coating Weights with the coating weight obtained from a ferric ion saturated solution.

The preferred ferric ion concentration in any of the solutions of this invention is the saturation value. When the solutions are saturated with the 'ferric ion and contain other constituents within the ranges above set forth, it has been found that the coating weight which is obtained varies directly with the temperature between about 110 F. and the boiling point, and this relationship holds true for contact time of the surface in excess of about 10-15 seconds. Most of the coating which forms is formed within the first 10-15 seconds and thereafter the coating weight remains about the same. As above indicated, different coating Weights are obtained on zinc surfaces obtained from different suppliers when identical solutions and application conditions are employed. For any given zinc material to be coated the above indicated direct variation of the resultant coating weight with temperature in the range of about 110 F. and the boiling point applies. It has also been observed that the solutions of this invention which contain the fluoride ion in the For example, the 26 point total acid solution form of silico fluoride ion rather than the fluoborate ion form consistently higher coating weights for a given P0 value when all other conditions are common, and this difference will be apparent from a consideration of the coating weight data which follows. It has been found that with solutions containing the silico fluoride ion within the concentration range above disclosed that the coating weight which is obtained from such a solution saturated with the ferric ion varies directly with the temperature with reference points as follows:

Coating weight,

Temperature, F.: mg./sq. ft.

When using solutions containing the fluoborate ion within the range above specified in a solution of this invention saturated with the ferric ion, the coating weight which is obtained is a direct function of the temperature with reference points as follows:

Coating weight,

Temperature, F.: mg./sq. ft. 34-94 50-110 47-167 63-183 80-200 101-221 122-242 The solutions of this invention may be applied to the surface to be coated by spraying, roller coating, by atomizing the solution on a preliminarily heated zinc surface or by dipping the part to be coated in a tank containing the solution. Solutions will form coatings in the range of about 120 F. to the boiling point of the solution but are preferably operated in the range of about 130 F. to 180 F. with the best overall results being obtained with solutions at about 150 F. The parts to be coated should be free of grease, dirt, and the like and may be prepared for coating by the use of conventional cleaning procedures and materials, namely, alkali cleaners, mild alkaline cleaners, acidic cleaners or the like. It has also been found to be advantageous to employ pretreatment solutions following the cleaning steps such as that disclosed in United States Patents 2,310,239, 2,874,081 and 2,884,351, which pretreatment solutions are of the general type which contain a condensed phosphate and a small quantity of the titanium or zirconium ion. After the coating is formed by application of the solution to this invention, it is advantageous, particularly in those cases in which the coated surface is to be subsequently painted, to rinse the coating in a dilute aqueous chromic acid solution of conventional constituency, for example, one containing about 0.025 to 0.1% CrO After such a final chromic acid rinse, the coatings have good resistance to corrosion prior to the application of paint and when painted have been found to be more resistant to cracking, chipping and peeling when the painted surface is deformed such as by forming to final desired shape in dyes, by bending or the like. This improvement will be apparent from a consideration of the examples which follow:

The following examples are intended to illustrate the compositions and methods of this invention in somewhat greater detail but it is to be understood that the particular ingredients, the proportions of ingredients, and the conditions of operation do not define the limits of this invention which have been hereinabove set forth. Percent concentration throughout this specification and claims refers to percent weight/volume.

5 EXAMPLE I An aqueous concentrate was prepared by admixing Parts:

pic I and adding thereto ferric nitrate in a quantity sufficient to produce a concentration in the range of about 0.0035 to 0.0042%.

A second bath constituting a portion of the control 108.5 75 2% 1 131 5 bath above described in Example I was placed in a suit- 25.5 42 Be HN0 able container to form coatings. 4" x 12" zinc panels,

27.0 ZnO. cleaned and processed in the identical fashion described 37.0 N1CO above in Example I were then spray coated with each 60.0 30% T1 5115; (hydrofiuo of these operating solutions at a temperature of 150 F.

S11E16 0 The solution substantially free of iron, designated Solution 70% A, was allowed to remain in contact with the surface for about seconds and the panel Was then rinsed and repre- Sumclent Water was adfied Produce total Welght sentative ones were checked for coating weight and the of 516. A control operating solution of this concentrate average coating weight was found to be about 300 min was then lilllllled with1 water to form an operating $0111 15 ligrams per square foot. The solution modified to conon 6121C upon ana ysis was found to co t 024% tain 0.00350.0042% ferric ion, designated Solution B, 5 l i 023% fluonde; 1% 023% N was allowed to remain in contact with the surface for 15 fernc and a tqtal,acld Yalue pomts' seconds and the coating weight determination showed i concentraflon 9 femc 1S f f to be that the panels had an average coating Weight of about residual or an impurity concentration since no iron was 200 mgjsq. ft intentionally added. Portions of this bath were then After drying a number of each type of coated panels modlfied by a ddmg fem: filtrate processed as above described was then spray painted amounts sufficient to produce concentrations of the ferric with a metallic blue alkyd paint commercially available ion 1n the resulting solutions of 0.0015%, 0.0026%, as ]OneS Dabney Paint No. 24422 to form a dry film and and these ccfmposmons were thickness of about 1 mil. After spraying the panels were used In companson the above spficlfied control cured in a circulating air oven at a temperature averaging Each of these solutlons was used to form coatings about FfOr about 1 minute on 12" X 4" m f hot dlp Zmc panels Whlch The panels were subjected to a deformation test dehad been prel minarily convent onally cleaned in an alkali Signed to measure the tendency of the Point to peel frae contamlag 5% of a tltamum matenal made as ture and crack as the degree of deformation is increased. described by United States Patent 2,874,081, the cleaner The tests were conducted on an apparatus having a bemg empltwed m a dilute form E (.mnce per gallon female die portion provided with a slot wide, vary- After cleaning, the panels were rinsed 1n hot Water and ing from the Surface level to a depth of of an then Sprayed Wlth each of abfwe qescflbed (fomposl' inch at a distance of 3" from the end of the slot at surface tions at 150 F. for the varying times indicated In Table level The panel to be tested is placed Over the female I belOW- Aftel: Procfissmg, Panels ,Were Yemoved, die and a /8 punch forces the panel into the slot to water rinsed, dr1ed and checked for coatlng weight. form an indentation f Unhane in cross section. The

Table I painted surface along the convex edges of the indentation is in the area of greatest tendency to peel, crack 40 and fracture and by placing the cellophane tape along this Ferric ion Coatmg mgJsq' convex edge and rapidly pulling the same from the panel, Solution concentra- O 30 any non-adherent paint is removed. These panels are percent ig 5% sen Sec rated numerically in terms of the percentage of the paint which is lost or removed along these convex edges. The 00001 365 480 520 505 490 515 45 panels were tested after 24 hours. Duplicate groups of 0.0055 165 280 310 gig these panels were permitted to age for 1 week, 2 weeks, 818812 38 i3? 210 215 190 4 weeks, 8 weeks and 26 weeks, and then tested on the 0.00 7 140 150 170 175 same apparatus. The results of these tests are set forth below, in tabular form, the letter capital P, referring It will be seen from an inspection of the above data to peelage.

Table II Coating Average of 1st Range of 1st weight, Fe present ratings ratings 1 wk. 2 wks. 4 wks. 8 Wks. 26 wks. mgJsq. it.

' N 2st P 50 7 P 707 P 7571 0 "8 /1 93 P. ifiiigfifijjljw 288 Y s om137?1 35% P 137%.. 10%; P ria--. 35;, P.

that a substantial reduction in coating weight occurs at Average of 1st ratings refers to the average of the ina ferric ion concentration between 0.0015 and 0.0001 and dividual 1st ratings which were observed and appear withthat as the ferric ion concentration increases toward its in the range of 1st ratings column, and is given to provide saturation value, the coating weight which is formed a single numerical basis for comparison with the detends to level out and become uniform in the range of ferred time ratings.

about 15 0200 milligrams per square ft.

EXAMPLE II An aqueous operating bath was prepared by diluting The above data demonstrate that the coating produced in the presence of the ferric ion having a coating weight less than that produced from the solution containing no ferric ion, is substantially superior with respect to peelage a portion of the bath described above in detail in Examof the painted surface.

7 EXAMPLE III This example illustrates the superiority of the coating formed from a composition including the ferric ion tion containing as its essential coating producing ingredients about 0.5% to about 4% phosphate ion, an ion of the group consisting of Zinc ion and the manganese ion in a proportion at least suflicient to form dihydrogen phosrelative to a coating of the same weight produced from 5 p ate With all said phosphate ion, about 0.01% to about a composition otherwise identical except that it includes 1% of at least n n selected from the group Consisting no f i i of the cobalt 1011 and the nickel ion, about 0.03% to about A series of 4" x 12 continuous hot dip panels, cleaned 1% of the fluonde not m thin} being and processed in an identical manner to those described gz as 51mph? fluonde 1on5, 531d fluorlde P P above in Example I, were coated with a portion of the i g from P cPnslstlng of control bath, described in Example I, designated Solution Home am uo crate SlhclJfluondF at o east about 0.0015% of the ferric ion, said solution having C, by spraying at 150 R, for a contact time of about 4 an acid p v y in the range of about 10 to about 110 total seconds Panels produqed by i procedure were found acid points, and having a temperature in the range of about to have an average coating weight of about 200 rug/sq. R to the boiling point of the Solution and main Another Seuss f 1dent1ca1 Panels were processed taining said solution in contact with said surface for at in the solution designated Solution B of Example 11, least about 10 Seconds under the Conditions Specified in that f f 6. A method for coating zinc surfaces which comprises After Coating and rinsing, and Palntlng Wlth the Same the steps of contacting said surface with an aqueous solu- Paint which was pp and Cured in the manner tion containing as its essential coating producing ingrescri ed above in Example 11, respective panels were dients about 0.5% to about 4% phosphate ion, zinc ion jected to the deformation test described above in Exin a proportion at least suflicient to form dihydrogen ample II. phosphate with all said phosphate ion, about 0.01% to The results of these tests are set forth in tabular form about 1% of the nickel ion, about 0.03% to about 1% of b l the fluoride ion, but not more than 0.5 being present as Table III Coating Average of 1st Range of 1st w h present ratings gs 1 wk. 2 \Vks. 4 wks. s \vks. 26 wks. rug/sq. ft.

Solution A 300 N0 P 75% P-- 78% 1 93% P, Shihiifiii: 383 N0 30% m /1.1 5 iizizz; it??? i 13: 2332 i 13: 32 1 31 Better gloss on the painted material results from use of Solution B, and noticeably less hazing than the painted panels from Solution A and Solution C.

What is claimed is:

1. A method for coating zinc comprising the step of treating the surface with an aqueous solution containing as its essential coating producing ingredients about 0.5 to about 4.0% phosphate ion, an ion of the group consisting of the zinc ion and manganese ion in a proportion at least sufficient to form dihydrogen phosphate With all said phosphate ion, about 0.01 to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, about 0.03% to about 1% of the fluoride ion but not more than 0.5% being present as simple fluoride ion, said fluoride ion being added as a material selected from the group consisting of hydrofluoric acid, fluoborate and silicofluoride, and at least about 0.0015 of the ferric ion.

2. A method for coating zinc which comprises the step of treating the surface with an aqueous solution containing as its essential coating producing ingredients about 0.5 to about 4% phosphate ion, zinc ion in a proportion at least sufficient to form dihydrogen phosphate with all said phosphate ion, about 0.01% to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, about 0.03% to about 1% of the fluoride ion, but not more than 0.5 being present as simple fluoride ions, said fluoride ion being added as a material selected from the group consisting of hydrofluoric acid, fluoborate and silicofluoride, and at least about 0.0015 of the ferric ion.

3. A method in accordance with claim 1 wherein said fluoride is present in the form of the silico fluoride ion.

4. A method in accordance with claim 1 wherein said fluoride is present in the form of the fluoborate ion.

5. A method for coating zinc surfaces which comprises the steps of contacting said surface with an aqueous solusimple fluoride ions, said fluoride ion being added as a material selected from the group consisting of hydrofluoric acid, fluoborate and silicofluoride, and at least about 0.0015 of the ferric ion, said solution having an acidity in the range of about 10 to about total acid points, and having a temperature in the range of about 110 F. to the boiling point of the solution, and maintaining said solution in contact with said surface for at least about 10 seconds.

7. A method for producing phosphate coatings on zinc surfaces having a preselected coating weight which comprises the steps of treating the said zinc surface with an aqueous solution containing as its essential coating producing ingredients about 0.5% to about 4% phosphate 1on, an ion of the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with all said phosphate ion, about 0.01% to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, about 0.03% to about 1% of the fluoride ion in the form of an ion selected from the group consisting of the silico fluoride ion and the fluoborate ion, and the ferric ion in a quantity sufiicient to saturate said solution, and adjusting the temperature of said solution to within the range of about 110 F. to the boiling point of said solution relative to the desired coating weight with reference points as follows:

Coating weight Temperature, F.: mg./sq. ft. 34-200 8. A method for producing phosphate coatings on zinc surfaces having a preselected coating weight which comprises the steps of treating the said zinc surface with an aqueous solution containing as its essential coating producing ingredients about 0.5% to about 4% phosphate ion, an ion of the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with all said phosphate ion, about 0.01% to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, about 0.03% to about 1% of the silicofiuoride ion, and the ferric ion in a quantity sufficient to saturate said solution, and adjusting the temperature of said solution to within the range of about 110 F. to the boiling point of said solution relative to the desired coating weight with reference points as follows:

9. A method for producing phosphate coatings on zinc surfaces having a preselected coating weight which comprises the steps of treating the said zinc surface with an aqueous solution containing as its essential coating producing ingredients about 0.5% to about 4% phosphate ion, an ion of the group consisting of the zinc ion and the manganese ion in a proportion at least sufiicient to form dihydrogen phosphate with all said phosphate ion, about 0.01% to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, about 0.03% to about 1% of the fluoborate ion, and the ferric ion in a quantity sufficient to saturate said solution, and adjusting the temperature of said solution to within the range of about 110 F. to the boiling point of said solution relative to the desired coating weight with reference points as follows:

Coating weight 10. In a method for forming phosphate coatings on zinc surfaces with an aqueous solution containing as its essential coating producing ingredients about 0.5% to about 4% phosphate ion, an ion of the group consisting of the zinc ion and manganese ion in a proportion at least sufficient to form dihydrogen phosphate with all said phosphate ion, about 0.01% to about 1% of at least one ion selected from the group consisting of the cobalt ion and the nickel ion, and about 0.03% to about 1% of the fluoride ion, the improvement which consists of modifying the coating forming ability of said solution such that the coating weight increases substantially linearly with temperature in the range of about F. to about F., which comprises incorporating in said solution sufficient ferric ion to saturate said solution, and thereafter forming a coating on said zinc surface at a temperature in the range of about 120 F. to about 180 F.

References Cited by the Examiner UNITED STATES PATENTS 2,346,302 4/1944 Hays et a1. 1486.15 2,479,564 8/1949 Gilbert 1486.15 2,487,137 11/1949 Hoover et a1 1486.15 2,813,812 11/1957 Sommers et al 148-6.15 2,835,617 5/1958 Mauer 1486.15

FOREIGN PATENTS 487,851 9/ 1936 Great Britain. 768,443 2/ 1957 Great Britain.

JOSEPH B. SPENCER, Primary Examiner.

Examiners. 

1. A METHOD FOR COATING ZINC COMPRISING THE STEP OF TREATING THE SURFACE WITH AN AQUEOUS SOLUTION CONTAINING AS ITS ESSENTIAL COATING PRODUCING INGREDIENTS ABOUT 0.5% TO ABOUT 4.0 PHOSPHATE ION, AN IRON OF THE GROUP CONSISTING OF THE ZINC ION AND MANGANESE ION INA PROPORTION AT LEAST SUFFICIENT TO FORM DIHYDROGEN PHOSPHATE WITH ALL SAID PHOSPHATE ION, ABOUT 0.01 TO ABOUT 1% OF AT LEAST ONE ION SELECTED FROM THE GROUP CONSISTING OF THE COBALT ION AND THE NICKEL ION, ABOUT 0.03% TO ABOUT 1% OF THE FLUORIDE ION BUT NOT MORE THAN 0.5% BEING PRESENT AS SIMPLE FLUORIDE ION, SAID FLUORIDE ION BEING ADDED AS A MATERIAL SELECTED FROM THE GROUP CONSISTING OF HYDROFLUORIC ACID, FLUOBORATE AND SILICOFLUORIDE, AND AT LEAST ABOUT 0.0015% OF THE FERRIC ION. 